COELENTERATES AND PLANARIANS 217 



latter, which have to be considered as possible factors responsible for the 

 regenerative and integrative changes setting in after an injury. 



Homoiogenous tissues can bring about such an equilibration, whereas after 

 transplantation of pieces belonging to a different species or genus, this result 

 is only temporary and after some time regenerative, integrative activity sets 

 in at the point of union, so that, as Goetsch observed, the two parts of two 

 organisms previously joined together separate again. This fact makes it rea- 

 sonable to assume that contact substances whose organismal differentials are 

 not too distant are needed for equilibration. Also, the inductive action, which 

 a transplant exerts on the mesenchymatous tissue of the host, takes place, 

 according to Gebhardt, only if transplant and host are homoiogenous ; if the 

 adjoining tissues carry heterodifferentials no induction is noted. On the other 

 hand, the organizing action of ganglionic material, studied by Santos, is 

 effective not only when both graft and host belong to the same species, but 

 also when they belong to different species; when the graft belongs, for in- 

 stance, to Planaria dorotocephala and the host to Planaria maculata, or vice 

 versa. 



As in hydroids, so also in planarians homoiotransplantations succeed readily 

 and distinct differences between the effects of auto- and of homoiotransplan- 

 tation have not been established; we have therefore no indications of the 

 existence of distinctive homoiodifferentials, though the existence of hetero- 

 differentials has been definitely demonstrated. As stated, in certain favorable 

 cases heterotransplants act like homoiotransplants ; but it seems, as a rule, 

 that contact and distance mechanisms active at the point of union between 

 two homoiogenous pieces are ineffective in heterogenous combinations. Hetero- 

 transplantation of small pieces seems not to lead to a complete union of the 

 graft with the surrounding host tissue. 



So far as the plasticity of organs and the lack of manifestation of finer 

 organismal differentials are concerned, there is thus a fargoing likeness in 

 hydroids and planarians, and the same general conclusions apply in these two 

 groups of animals as to the similarity of the mechanisms which maintain the 

 intraorganismal equilibrium and the absence of distinctions between autog- 

 enous and homoiogenous tissues in this equilibrium. Various types of organ- 

 izer and regenerative, integrative influences play a role in determining the 

 mutual relations of organs and tissues in these lower organisms, all tending 

 to reestablish the original equilibrium when it is disturbed. It is this autog- 

 enous equilibrium, as we have studied it in higher organisms and as it exists 

 in a wider sense also in these primitive organisms, which determines the 

 maintenance of individuality. However, here the type of interactions between 

 organs and tissues which helps to sustain the normal equilibrium, or to re- 

 establish a disturbed equilibrium, is in some respects more accessible to 

 analysis than are the corresponding mechanisms in the higher organisms. In 

 the latter, accompanying the greater refinement in individuality, the means of 

 restoring an unbalanced equilibrium, in the sense in which it can be accom- 

 plished in these very primitive organisms, are lacking. 



